1. Field of the Invention
The present invention is directed to an x-ray tube of the type having a vacuum housing, a cathode arrangement in the housing that emits electrons and an anode in the housing with a target surface on which the electrodes, accelerated by an electrical field and forming an electron beam, are incident in a focal spot, and having a quadrupole magnet system, including a coil, for the focusing and deflection of the electron beam.
2. Description of the Prior Art
An x-ray tube of the above general type is known for example from German OS 196 31 899. X-ray tubes of this type of construction, or of a comparable type of construction, are used both in medicine and outside of medicine, e.g. for material examinations.
Medical areas of application of x-ray tubes of this type are, for example, in the fields of neuroradiography, general angiography and cardiology. In comparison to other medical areas of application, these medical areas of application are distinguished in that a spatial perception (i.e., an image with depth), for example, the path of vessels, in the body of a patient to be examined is desired, which can be achieved by means of stereo exposures of the relevant body area of the patient. The term xe2x80x9cstereo exposures,xe2x80x9d as used herein means that the body region to be examined is irradiated from at least two different x-ray projection angles one after the other, and the results are displayed on a divided image reproduction device or on two image reproduction devices. In the observation of the items of the image information shown on a divided image reproduction device or on two image reproduction devices, a spatial impression is seen by a viewer.
It is known to execute such stereo exposures
a) with an x-ray tube R1 that is displaced in linear fashion between two positions (cf. FIG. 1a),
b) with an x-ray tube R2 that is rotated around a point of rotation (cf. FIG. 1b),
c) with two x-ray tubes R3, R4 (cf. FIG. 1c) arranged next to one another, or
d) with a multi-cathode x-ray tube R5, having, for example, three cathodes K1, K2, K3 (cf. FIG. 1d).
Solutions a) and b) have the disadvantage that the image exposure frequency is too low for x-ray motion picture (cinxc3xa9) exposures. Solution c) has the disadvantage that it is expensive due to requiring two x-ray tubes, and the stereo basis, i.e., the spacing of the foci of the x-ray tubes, is too large. Solution d) is indeed suitable for all application techniques in stereo exposures, but the construction of the x-ray tube with respect to the multi-cathode arrangement is technically complicated and thus expensive.
From U.S. Pat. No. 4,993,055, a rotating tube is known in which two focal spots can be produced, so that the rotating tube is also suitable for stereo exposures. In order to deflect the electron beam running from the cathode to the anode, the rotating tube has two groups of two magnet coils (i.e., tow magnet coils per group) opposed to one another that produce a substantially homogenous magnetic field. The groups of magnet coils are arranged so as to be offset from one another by a particular angle of rotation, the angle of rotation substantially corresponding to the angle at which the two focal spots are offset. Given activation of one group of coils, the electron beam is thus deflected onto one focal spot, and given activation of the other group of coils, it is deflected onto the other focal spot.
A disadvantage of this known system is that a pair of coils is required for each displacement of the focal spot, making the construction of the rotating tube, in particular relating to the arrangement of the magnet coils, relatively expensive.
From U.S. Pat. No. 4,607,380, an x-tube is known with two magnets arranged one after the other, of which one magnet serves for the deflection of the electron beam and the other for the focusing of the electron beam.
In German OS 34 01 749, an x-ray tube is disclosed that has deflecting electrodes, arranged one after the other, for an electron beam.
An object of the present invention is to provide an x-ray tube of the type initially described wherein the focal spot of the x-ray tube can be displaced and the x-ray tube is technically simple to manufacture and is of an economical construction.
According to the invention, this object is achieved in an x-ray tube with a vacuum housing containing a cathode arrangement that emits electrons and an anode with a target surface on which the electrons, accelerated by an electrical field and forming an electron beam, are incident in a focal spot, and having a quadrupole magnet system, including a coil, for the focusing and deflection of the electron beam, and a control unit allocated to the quadrupole magnet system, with which several different parameter sets of coil currents can be stored and activated, the coil current sets being predetermined to cause the focal spot to be displaced in azimuthal fashion onto particular locations of the target surface of the anode, depending on the parameter set which is activated. The x-ray tube thus has only a single quadrupole magnet system, provided both for focusing and for deflecting the electron beam. The control unit allocated to the quadrupole magnet system makes it possible, by predetermination, storing and activation of various parameter sets of coil currents for the coils of the quadrupole magnet system, to displace the focal spot of the x-ray tube discretely, in azimuthal fashion, onto particular locations of the target surface of the anode, while maintaining the relative position of the quadrupole magnet system to the x-ray tube. A dipole field that serves for the deflection of the electron beam is thereby superposed or a quadrupole field that serves for the focusing of the electron beam, the quadrupole field being produced by coil current components that are substantially equal in magnitude, and the dipole field is produced, according to the desired position of the focal spot, by coil current components whose magnitudes are not necessarily equal. The coil current components are respectively added to one another to form a total coil current allocated to a coil of the quadrupole magnet system. Given a quadrupole magnet system with four coils, four coil currents, each individually allocated to one coil of the quadrupole magnet system, form a parameter set for the production of a particular focal spot. Due to the use of only one quadrupole magnet system provided with a control unit for the focusing and deflection of the electron beam, the inventive x-ray tube is of relatively simple construction, and thus can be manufactured in a cost-advantageous manner.
In a preferred embodiment of the invention the x-ray tube has at least one coil connected spatially downstream from the quadrupole magnet system, and with this coil a magnet field can be produced with which the shape of the focal spot and its orientation relative to the target surface of the anode can be influenced. The coil can be a solenoid. The magnetic field produced by the solenoid serves to influence the electron beam after this beam has traversed the magnetic field of the quadrupole magnet system, i.e., the quadrupole field and dipole field are superimposed. This is because in many parameter sets of coil currents that effect a particular deflection of the electron beam onto an azimuthally displaced focal spot of the anode, due to non-homogeneities of the resulting magnetic field at the location at which the electron beam passes through the magnetic field of the quadrupole magnet system an undesired spreading of the electron beam results and thus an undesired spreading of the displaced focal spot would occur, and the resolution capacity of an x-ray exposure would be degraded. This undesired spreading of the focal spot can be counteracted by means of a suitable magnetic field that influences the electron beam, so that a focal spot of the desired length and width advantageously arises on the target surface of the anode. There is also the possibility of rotating the focal spot under the influence of the magnetic field, i.e., modifying the orientation of the focal spot relative to the target surface so that, given a displaced focal spot, the focal spot can always be oriented in such a way that x-ray exposures with high resolution capacity can be produced.
If the inventive x-ray tube is, for example, a fixed-anode x-ray tube or a rotating-anode x-ray tube, provided for stereo exposures of subjects or for material investigations, then according to a further version of the invention the vacuum housing of the x-ray tube can have at least two radiation exit windows respectively allocated to different focal spots. An inventive x-ray tube with several (e.g. four) beam exit windows, each allocated to a focal spot, is for example of great interest for industrial diagnostic purposes, e.g. checking soldered connections on circuit boards, since with only one such x-ray tube in a test stand test samples can continuously be supplied to the test stand from several sides, namely the x-ray exit sides of the x-ray tube, and the test samples can be irradiated, i.e. tested, one after the other in a very short time, with the focal spot being azimuthally displaced corresponding to the defined position of the test sample relative to the x-ray tube.
In a further embodiment of the invention the vacuum housing has an annular beam exit window. This is preferably the case if the x-ray tube is a rotating tube, i.e., the vacuum housing of the x-ray tube can be rotated around an axis, with the cathode arrangement and the anode are respectively connected fixedly with the vacuum housing. The inventive construction of such a rotating tube with a quadrupole magnet system having a control unit for the displacement of a focal spot, the rotating tube, can be used for stereo exposures of subjects.